Electrically insulated metallic surfaces with interior corners and methods and compositions therefor

ABSTRACT

An effective electrically insulating film on the surface of a metal object, such as a common type of electric motor core assembly, in which the surface includes an interior comer on which insulation is desired, can be formed by autodeposition with an adequate thickness in the interior corner without need for excessive thickness on other parts of the surface that are more readily covered by prior art methods of applying an insulating coating. If the autodeposition composition used includes as its primary film-forming component a copolymer of certain acrylic monomers, a very high volume resistivity can be achieved.

FIELD OF THE INVENTION

This invention relates to (i) methods for coating, with electricallyinsulating films, metallic surfaces with interior corners, (ii) articlesof manufacture so produced, and particularly (iii) electric motorscontaining them. More particularly, this invention relates to methodsthat can form an insulating film with excellent electrical insulatingproperties on the surface of a sheet-steel-laminated motor core bybringing the surface of such a motor core into contact with anautodepositing waterborne coating composition.

DESCRIPTION OF RELATED ART

Many motor cores used, for example, in small motors, require electricalinsulation between electrically conductive core elements and distinctelectrically conductive windings in close proximity to the coreelements. The insulation has generally been provided by an insulatingtreatment of the core elements.

This insulating treatment has heretofore consisted of forming aninsulating layer on the motor core surface using electrodepositioncoatings, solvent-based sprays, powder paints, and the like. However,insulating layers applied using the aforesaid paints have a pronouncedtendency to debond at the corners of the motor core, and the occurrenceof this debonding causes these insulating layers to suffer from adiminished insulating performance. This has required that the insulatinglayer be thick.

Recent requirements on motors have been for smaller size, thinnerconfigurations, higher performance (higher withstand voltages), andthinner insulation.

Japanese Patent Application Laid Open [Kokai or Unexamined] Number Hei5-300681 [300,681/1993] discloses a technology for reducing thethickness of the insulation. This reference teaches the formation ofinsulation with excellent insulating properties and an excellentcorner-coating capacity. The insulation described therein comprises twolayers with a total thickness of 50 to 80 micrometres and is formed bycoating the motor core surface with an epoxy resin functioning as primerand then with a ceramic paint functioning as top coat. This referencestates simply that the insulating layer should consist of multiplelayers; it is not limited to two layers and may consist of a largernumber, such as three or four layers. This technology requires at leasttwo coating operations to form the insulating layer and thus suffersfrom the drawback of poor productivity.

PROBLEMS TO BE SOLVED BY THE INVENTION

A major object of the present invention is to provide a method forcoating the surface of laminated motor cores, or other surfaces withinterior corners, with an electrical insulating film wherein said methodcan shorten the insulating treatment operation, can provide a thinnerelectrical insulating layer than heretofore available, and/or canprovide a surface that has a coating with excellent electricalinsulating properties, in particular, a high-quality electricalinsulating layer in the corners of the surface.

SUMMARY OF THE INVENTION

It has been found that the desired thinner, high-quality insulating filmcan be generated in a shorter insulating treatment operation bycontacting a surface, especially that of a laminated motor core, with anautodepositing waterborne coating composition comprising acoating-forming resin emulsion, acid, oxidizing agent, metal ion, andwater, and by thereafter drying by heating.

In specific terms, the present invention provides a method for coating asurface with at least one interior corner, preferably a corner formed byan intersection between a substantially planar surface and a convexarcuate surface, so that the corner includes a deeper recess than wouldbe formed by intersection between two mutually perpendicularsubstantially planar surfaces, with an electrical insulating film,characterized by forming an electrical insulating film on the surface bysteps of:

(I) depositing an uncured resin film on said surface by bringing thesurface into contact with an autodepositing waterborne coatingcomposition comprising, or more preferably consisting essentially of, acoating-forming resin emulsion, acid, oxidizing agent, metal ions, andwater; and then

(II) drying the said uncured resin film, while it remains in place onthe surface, by heating.

The present invention also provides an alternative method for coatingsuch a surface with an electrical insulating film, characterized byforming an electrical insulating film on the surface by steps of

(I) depositing an uncured resin film on said surface by bringing thesurface into contact with an autodepositing waterborne coatingcomposition comprising, or more preferably consisting essentially of, acoating-forming resin emulsion, acid, oxidizing agent, metal ions, andwater;

(II) contacting said uncured resin film with an aqueouschromium-containing solution; and then

(III) drying said uncured resin film, while it remains in place on thesurface, by heating.

Other embodiments of the invention include liquid autodeposition coatingcompositions especially adapted for use in a process according to theinvention and articles of manufacture including a surface coated by aprocess according to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of a typical shape formed from metal sheeting foruse in making a motor core. Several such substantially identical shapesare laminated together in a pressing operation to form part of thedesired core, which, in a top view, has the same shape as the singlesheets from which it is formed. The core has a plural number ofprojecting poles 1, and the distal ends 2 of projecting poles 1 haveconvex circularly arcuate surfaces. When several such shaped sheets havebeen laminated together, each projecting pole has four substantiallyplanar surfaces: a top (the only one visible in FIG. 1), a bottom, andtwo sides. The core also has interior convex circularly arcuate surfaces5 between the projecting poles 1

FIG. 2 is a sectional partial view, on a larger scale than FIG. 1, ofthe area between two of the projecting poles 1 in FIG. 1 and of theimmediately surrounding parts of these projecting poles and of othermaterials which are part of a completed core and are at least partiallysituated between each pair of poles. (In a conventional completed core,the area between each pair of poles is substantially identical). Eachprojecting pole 1 is wound with windings 3. In the absence of insulationbetween the projecting poles 1 and windings 3, the windings would shortcircuit to each other through the core and the motor would not function.Therefore, insulation 4 is required between the core and the windingsand is generally provided by a treatment of the laminated core beforethe windings are applied.

DETAILED DESCRIPTION INCLUDING PREFERRED EMBODIMENTS

The invention will be described below primarily with reference to motorcores of the type noted above. However, the invention is useful, mutatismutandis, for other applications in which thin insulating layers onmetal substrate surfaces that include inner corners, particularly suchcorners as are formed by intersections between substantially planar andconvex arcuate surfaces, are needed.

The invention is applied to motor cores fabricated by laminating aplural number of metal sheets. While the metal used for this purpose isnot critical, sheet steel is ordinarily used. The method for laminatingthe sheet steel is also not critical and techniques such as pressoperations and the like may be used.

The autodepositing waterborne coating composition used in the presentinvention contains a coating-forming resin emulsion, acid, oxidizingagent, metal ions, and water, and may also contain optional components.

The resin in the coating-forming resin emulsion used by the presentinventions is exemplified by acrylic resins, vinyl chloride resins,vinylidene chloride resins, urethane resins, epoxy resins, and polyesterresins. The resin used by the present invention may also be a mixture ofany combination of the aforementioned resins.

Acrylic resins are particularly preferred and are exemplified by thehomopolymers and copolymers prepared from (meth)acrylate ester monomers,(meth)-acrylic acid-type monomers, styrene, ethylene, and the like. Thecopolymers consist of two or more selections from such monomers. The(meth)acrylate ester monomers are exemplified by methyl acrylate, ethylacrylate, n-butyl acrylate, 2-hydroxyethyl acrylate, 2-hydroxypropylacrylate, 2-ethylhexyl acrylate, methyl methacrylate, ethylmethacrylate, n-butyl methacrylate, 2-hydroxyethyl methacrylate,2-hydroxypropyl methacrylate, glycidyl acrylate, glycidyl methacrylate,etc. The (meth)acrylic acid-type monomers are exemplified by acrylamide,methacrylamide, acrylonitrile, acrylic acid, and methacrylic acid.

Polymers of a mixture of monomers including at least one selection fromeach of the following groups are preferred over other types of acrylicpolymers:

(A) acrylic and methacrylic acids;

(B) esters of acrylic and methacrylic acids with alcohols containingfrom 1 to 4 carbon atoms per alcohol molecule; and

(C) acrylonitrile, methacrylonitrile, acrylamide, and methacrylamide.Preferably, the mixture of monomers also includes:

(D) “internal surfactant” type molecules as described in U.S. Pat. No.5,352,726 of Oct. 4, 1994 to Hall from column 7 line 59 through column 9line 28, which portion of said patent is hereby incorporated herein byreference, except to any extent that it may be inconsistent with anyexplicit statement herein.

The ratio by weight in a preferred acrylic copolymer resin of residuesfrom components (A) through (C) as defined above preferably is, withincreasing preference in the order given, 1.0:{10-80}:{3-50},1.0:{15-65}:{6-40}, 1.0:{24-50}:{8-25}, or 1.0:{30-38}:{12-18} for theratio (A):(B):(C). Independently, when component (D) is used, its amountpreferably is, with increasing preference in the order given, 0.2-5.0,0.4-2.5, 0.6-1.8, or 0.8-1.2% by weight of the total of all the othermonomers used in the monomer mixture.

Still more preferably, component (B) of monomers as defined abovepreferably includes at least one selection from each of the followingsubcomponents:

(B.1) esters of methacrylic acid with alcohols having 1, 2, or 3, mostpreferably 1, carbon atoms per molecule of alcohol; and

(B.2) esters of acrylic acid.

When both subcomponents (B.1) and (B.2) are present, the ratio by weightof (B.2) to (B.1) preferably is, with increasing preference in the ordergiven, {0.7-2.5}:1.0, {1.0-1.8}:1.0, or {1.3-1.5}:1.0. Yet morepreferably, subcomponent (B.2) includes at least one selection from eachof the following sub-subcomponents:

(B.2.1) esters of acrylic acid with ethyl and/or methyl alcohol; and

(B.2.2) esters of acrylic acid with propyl and/or butyl alcohol.

When both sub-subcomponents (B.2.1) and (B.2.2) are present, the ratioof (B.2.1):(B.2.2) preferably is, with increasing preference in theorder given, 1.0:{0.3-3.0}, 1.0:{0.5-2.0}, 1.0:{0.7-1.5}, or 1.0:{0.85-1.15}.

The molecular weight of the acrylic resin is not normally critical.Preferably, however, the acrylic resin has a molecular weight of 50,000to 1,000,000, or preferably 100,000 to 1,000,000, as determined by gelpermeation chromatography in tetrahydrofuran, using polystyrene orpolyacrylate ester standards.

The coating-forming resin emulsion used by the present invention willtypically be a resin emulsion as directly afforded by the usual emulsionpolymerization methods. Also usable, however, are the resin emulsionsafforded by the emulsification and dispersion in water of resin alreadyprepared by various polymerization methods.

In the case of resin emulsions prepared by emulsion polymerization, thepolymerization conditions are not normally critically different from theusual procedures known in the art, which therefore are preferablyemployed. One example of the preparation of a coating-forming resinemulsion comprises running a polymerization reaction in a mixture of atleast water, anionic and/or nonionic surfactant, polymerizationinitiator, and monomer (resin component) as described above.

The acid used in the present invention can be, for example, at least oneselection from fluozirconic acid, fluotitanic acid, fluosilicic acid,fluoboric acid, phosphoric acid, nitric acid, and the like, buthydrofluoric acid is preferred.

The oxidizing agent used in the present invention is exemplified byhydrogen peroxide, potassium permanganate, sodium nitrite, and the like;hydrogen peroxide is preferred.

The source compound for supplying the metal ions used by the presentinvention is not normally critical, but this compound must be stable inthe subject coating composition. Examples of the source compound areferric fluoride, ferric nitrate, ferrous phosphate, cobaltous nitrate,and the like; ferric fluoride is preferred.

The resin content in the autodepositing waterborne coating compositionused by the present invention is preferably from 5 to 550 grams perliter (hereinafter usually abbreviated as “g/L”) and more preferablyfrom 30 to 100 g/L, in each case measured as the resin solidsconcentration.

Also, independently for each concentration noted, in an autodepositingwaterborne coating composition used by the present invention: The acidconcentration is preferably from 0.1 to 5.0 g/L and more preferably from0.5 to 3.0 g/L; the oxidizing agent concentration is preferably from0.01 to 3.0 g/L and the concentration of the metal ion source compoundis preferably from 0.1 to 50, more preferably from 0.5 to 20, still morepreferably from 1.0 to 10, and yet more preferably from 1.5 to 4, g/L.

As an optional component, the autodepositing waterborne coatingcomposition used in the present invention may also contain afilm-forming aid for the purpose of reducing the minimum film-formingtemperature and facilitating fusion and adhesion of the deposited resinparticles. This film-forming aid is exemplified by trialkylpentanediolisobutyrate, the alkyl Carbitols™, and the like. Pigment is anotheroptional component that may be present in the composition, for example,carbon black, phthalocyanine blue, phthalocyanine green, quinacridonered, Hansa yellow, benzidine yellow, and the like.

Dipping, spraying, etc., preferably dipping, can be employed to bringthe autodepositing waterborne coating composition used in the presentinvention into contact with the surface of the laminated motor core.Neither the treatment temperature nor the treatment time areparticularly critical, but suitable conditions for dipping are dippingfor 30 to 300 seconds and preferably for 60 to 180 seconds in thecomposition maintained at ambient temperature, for example, 18° C. to25° C. Any surfaces not desired to be coated, such as the distal endsurfaces 2 shown in FIG. 1, may be protected with wax, tape, plasticfilm, or some similar temporary protective coating as generally known inthe autodeposition art.

The resin film add-on by the coating composition to the surface of thelaminated motor core is not always critical, but post-drying filmthicknesses of 10 to 40 micrometres are usually preferred.

In addition, the motor core surface is ordinarily cleaned by degreasingand water rinsing before application of the coating composition. Resindeposition on the motor core surface is generally followed by a waterrinse. This water rinse can be carried out by placing the core in awater flow, but is ordinarily run by dipping in water at ambienttemperature for 10 to 120 seconds.

The thermal drying step is not specifically restricted, but suitableconditions are 5 to 60 minutes and preferably 10 to 20 minutes in aforced convection oven at an atmosphere temperature of 80° C. to 180° C.

The usual hexavalent chromium compounds, such as dichromic acid,ammonium dichromate, and the like, are examples of the chromium employedin the chromium-containing aqueous solution optionally used by thepresent invention. The chromium concentration in this aqueous chromiumsolution is preferably from 0.1 to 20 g/L as hexavalent chromium. Theadditional presence of trivalent chromium in this aqueous chromiumsolution is unproblematic in terms of solution performance. Suitableconditions for contacting the uncured resin film with thechromium-containing aqueous solution are dipping for 30 to 180 secondsin the solution at ambient or elevated temperature. After this treatmentthe uncured resin film—without a water rinse—is dried by heating usingthe above-described conditions to give the electrical insulating film.Film formation according to the method of the present invention mayoptionally be followed by coating with a powder paint or a conventionalpaint.

During the formation of the uncured resin film on the sheet steellaminate, the coating method according to the present invention is ableto form an almost entirely uniform film in those regions where thesubject autodepositing coating composition comes into contact with thesheet steel surface. The present coating method is able to do thisbecause it achieves film formation through the chemical activity of theautodepositing coating composition over the metal work-piece surface(the metal ions eluted from the metal surface by etching act on theresin particles in the coating composition to cause their depositiononto the metal surface) without the use of an external electrical sourceas in electrodeposition. In addition, the character of the cornercoating of the sheet steel surface remains excellent even after thermaldrying of the uncured resin film, which results in the formation of aninsulating film with excellent insulating properties. In particular, thecomposition is preferably selected so that the dried resin film in athickness of 15 micrometres has a volume resistivity that is at least,with increasing preference in the order given, 30×10¹⁵, 5.0×10¹⁵,7.0×10¹⁵, 9.0×10¹⁵, or 10.0×10¹⁵ ohm centimeters.

A coating composition and process using it according to this inventionare preferably selected so that the thickness of the dried insulationfilm formed by the process, on a surface including both at least oneinterior corner and at least one substantially planar surface, in theinterior corner of the surface coated having the deepest recess (or inone of such corners if there are a plurality of such interior cornerswith equally deep recesses) has a thickness that is at least, withincreasing preference in the order given, 75, 80, 85, 90, 95, or 100% ofthe thickness of the insulation in a direction perpendicular to asubstantially planar part of the same coated surface coated in the sameprocess.

The invention will be explained in greater detail below through workingand comparative examples.

EXAMPLES

The following methods were used to evaluate the properties is reportedbelow, unless otherwise stated below.

Coating Thickness

A test motor core specimen as shown in FIG. 1 was cut across two of theprojecting poles 1 at a position that was interior to distal ends 2 andaccordingly intended to be covered by windings 3 as shown in FIG. 2before the core is used in an actual motor. The insulation thickness wasmeasured on part of an insulation-covered planar surface of the pole, asexemplified by dashed line X—X in FIG. 2, and in an interior cornerwhere the same planar surface meets an interior arcuate surface asexemplified by dashed line Y—Y in FIG. 2.

Appearance

After coating, a test specimen was visually inspected in order tomeasure the number of defects, e.g., swelling of the coating, cracking,and poor hiding by the film. The result of this inspection is reportedbelow on the following scale:

++: no defects (swelling, cracking, or poor hiding);

++: at least one but fewer than five defects;

x: 5 or more defects.

Insulation Performance Test

This test evaluated the practical quality of the insulation of theportion of the motor core over which the conductive windings are placed.The test specimen had 48 distinct areas for testing, one on each of thefour sides of each projecting pole 1 as shown in either of the drawingfigures. The coating at the center of a coated test specimen was peeledoff, and the thus-bared center was connected to the negative pole of atest needle. The positive pole of the test needle was scanned over thewinding region of the motor core in order to determine the number a ofdistinct areas along any portion of which current leakage occurred. Theinsulation performance was evaluated through the defect ratio=a/48and/or by the percentage equivalent of this defect ratio=100a/48. Themeasurement instrument was an insulation resistance meter Model FI-901from Nippon Technat Co., Ltd., using an applied voltage of 500 volts(hereinafter usually abbreviated as “V”).

Measurement of the Volume Resistivity

1. Method: according to Japanese Industrial Standard K 6911.

2. Measurement instrument: Advantest™ R8340A Ultra High Resistance Meterwith an Advantest™ R12704 Resistivity Chamber as electrode.

3. Measurement method and conditions: The test specimen was installedand the volume resistivity of the coating in ohm-cm was measured usingthe following procedures:

i. 0 externally applied V during 30 seconds (hereinafter usuallyabbreviated as “sec”) of discharging

ii. 500 externally applied V during 60 sec of charging

iii. 0 externally applied V during 30 sec of discharging.

Preparation of the Coating-forminc Resin Emulsion

A monomer mixture of 2 parts (denoting weight parts here and below) ofmethacrylic acid, 28 parts of methyl methacrylate, 30 parts ofacrylonitrile, 20 parts of ethyl acrylate, and 20 parts of butylacrylate was mixed with 1.0 part of acrylate ester-type reactivesurfactant (i.e., 1.0 weight % based on the total weight of theabove-listed five monomers), 0.3 part of ammonium persulfate, and 399.6parts of water. Emulsion polymerization was then run for 4 hours at 75°C. by the usual method to yield resin emulsion with 20% of resin solids.The resin was cooled to 40° C., and its pH was adjusted with 25% aqueousammonia to from 5 to 8 to give the coating-forming resin emulsion.

Constituents of Sample Autodepositing Waterborne Coating Composition (I)

Component Concentration in g/L the above-described 280   coating-forming resin emulsion film-forming aid A 4.00 hydrofluoric acid0.70 ferric fluoride 3.00 hydrogen peroxide 0.10 + deionized water tomake a total of 1 L.

The film-forming aid A consisted of trialkylpentanediol isobutyratemolecules. Its addition gave a minimum film-forming temperature of about20° C. for the composition.

Example 1

The test specimens were motor cores as shown in FIG. 1 that had beenfabricated by laminating a plural number of magnetic steel sheets bypressing. These were subjected to a preliminary cleaning. Theautodepositing waterborne coating composition prepared using the recipegiven above was maintained at a bath temperature of 20° C. to 22° C.,and the test specimens were coated by dipping for 60 seconds. This wasfollowed by a water rinse, by dipping in deionized water for 60 seconds,and then drying for 20 minutes in a hot-air oven at 180° C. The testspecimens were subjected to the various coating performance tests.

Example 2

While the autodepositing waterborne coating composition described forExample 1 was maintained at a bath temperature of 20° C. to 22° C.,preliminarily cleaned test specimens as described for Example 1 werecoated by dipping for 120 seconds. This was followed by a water rinse bydipping in deionized water for 60 seconds and then drying for 20 minutesin a hot-air oven at 180° C. The test specimens were subjected to thevarious coating performance tests.

Example 3

While the autodepositing waterborne coating composition described forExample 1 was maintained at a bath temperature of 20° C. to 22° C.,preliminarily cleaned test specimens as described for Example 1 werecoated by dipping for 180 seconds. This was followed by a water rinse bydipping in deionized water for 60 seconds and then drying for 20 minutesin a hot-air oven at 180° C. The test specimens were subjected to thevarious coating performance tests.

Example 4

While the autodepositing waterborne coating composition described forExample 1 was maintained at a bath temperature of 20° C. to 22° C.,preliminarily cleaned test specimens as described for Example 1 werecoated by dipping for 60 seconds. After a water rinse by dipping indeionized water for 60 seconds, the test specimens were dipped for 60seconds in an aqueous solution that contained 4 g/L of hexavalentchromium. This was followed, without subjecting the coating to anotherwater rinse, by drying for 20 minutes in a hot-air oven at 180° C. Thetest specimens were subjected to the various coating performance tests.

Example 5

While the autodepositing waterborne coating composition described forExample 1 was maintained at a bath temperature of 20° C. to 22° C.,preliminarily cleaned test specimens as described for Example 1 werecoated by dipping for 90 seconds. After a water rinse by dipping indeionized water for 60 seconds, the test specimens were dipped for 60seconds in an aqueous solution that contained 4 g/L of hexavalentchromium. This was followed, without subjecting the coating to anotherwater rinse, by drying for 20 minutes in a hot-air oven at 180° C. Thetest specimens were subjected to the various coating performance tests.

Example 6

While the autodepositing waterborne coating composition described forExample 1 was maintained at a bath temperature of 20° C. to 22° C.,preliminarily cleaned test specimens as described for Example 1 werecoated by dipping for 180 seconds. After a water rinse by dipping indeionized water for 60 seconds, the test specimens were dipped for 60seconds in an aqueous solution that contained 4 g/L of hexavalentchromium. This was followed, without subjecting the coating to anotherwater rinse, by drying for 20 minutes in a hot-air oven at 180° C. Thetest specimens were subjected to the various coating performance tests.

Comparative Example 1

The surfaces of preliminarily cleaned test specimens were coated with aprimer (II) (main component=epoxy resin) to a thickness of about 20micrometres on the planar regions of the core element on which windingsare later to be placed. This was followed by drying for 20 minutes in ahot-air oven at 200° C. The test specimens were subjected to the variouscoating performance tests.

Comparative Example 2

The surfaces of preliminarily cleaned test specimens were coated with aprimer (main component=epoxy resin) to a thickness of about 20micrometres on the planar regions of the core element on which windingsare later to be placed. This was followed by drying for 20 minutes in ahot-air oven at 200° C. The test specimens were removed from the ovenand a ceramic paint, consisting of silicate-modified polyether resin,was coated on the primer to a thickness of about 40 micrometres. Thiswas followed by drying for 20 minutes in a hot-air oven at 220° C. Thetest specimens were subjected to the various coating performance tests.

Example 7

The following test was run in order to compare the insulating propertiesof acrylic resin emulsions with those of vinylidene chloride resinemulsions.

The test specimens in each test were preliminarily cleaned cold-rolledrectangular steel sheet (Type SPCC, 70×100×0.8 mm). The test specimenswere coated by dipping in autodepositing waterborne coating compositionA or B maintained at a bath temperature of 20° C. to 22° C. Theconstituents for compositions A and B. except for water whichconstituted the balance of both compositions, are given in Table 1below. After dipping, the test specimens were rinsed by dipping indeionized water for 60 seconds and then dried in a hot-air oven. Thevolume resistivity in ohm-cm was then measured.

TABLE 1 Concentration of Ingredient, g/L, in Composition: Ingredient A BAcrylic resin emulsion as in Example 1 280 none Poly{vinylidenechloride} resin none 140 emulsion Film-Forming Aid A 4 none HydrofluoricAcid 0.7 0.7 Hydrogen Peroxide 0.1 0.1 Ferric Fluoride 3 3

Results for Examples and Comparison Examples, except for Example 7, thatdid not include the use of a hexavalent chromium-containing rinse of thewet autodeposited coating are given in Table 2, results for Examplesthat did include the use of a hexavalent chromium-containing rinse ofthe wet autodeposited coating are given in Table 3, and results forExample 7 are given in Table 4.

TABLE 2 Value of Measurement for: Example Number: Comp. Ex. #:Measurement Type and Units 1 2 3 1 2 Film thickness in micrometers:Along Line X-X: 12 20 32 22 58 Along Line Y-Y: 15 25 36  6 22 AppearanceRating: + + + +  +  x + + Insulation Defect Percentage:  8  0  0 75  8Abbreviation in Table 2 “Comp. Ex. #” means “Comparative Example Number.

TABLE 3 Value of Measurement for: Example Number: Measurement Type andUnits 4 5 6 Chromium Add-on, mg/m² 500  450  520  Film thickness inmicrometers: Along Line X-X: 12 21 33 Along Line Y-Y: 14 25 37Appearance Rating: + + + + + + Insulation Defect Percentage:  0  0  0Abbreviation in Table 3 “mg/m²” means “milligrams of stoichiometricequivalent as chromium metal added-on per square meter of surfacecoated.”

TABLE 4 Film Thickness in Micro- Volume Resistivity Value for FilmFormed from: meters Composition A Composition B 15 1.653 × 10¹⁶ ohmcentimeters 3.060 × 10¹⁵ ohm centimeters 20 3.002 × 10¹⁶ ohm centimeters3.143 × 10¹⁵ ohm centimeters 25 4.797 × 10¹⁶ ohm centimeters 3.794 ×10¹⁵ ohm centimeters

The following conclusions can be drawn based on Examples 1 to 7 andComparative Examples 1 and 2:

(1) Examples 1 to 3, which employed a coating method according to thepresent invention, provided a safe coating thickness in the cornerdespite a relatively small coating thickness overall, and thus providedexcellent insulating properties even at small coating thicknesses.

(2) In contrast to the preceding, a poor insulation performance wasobtained in Comparative Example 1, in which coating was executed by aconventional method. This required that the coating thickness beincreased as in Comparative Example 2 in order to obtain a satisfactoryinsulation performance.

(3) As demonstrated in Examples 4 to 6, the addition of chromium to theresin film according to the present invention can improve filmappearance for thick films and secure satisfactory insulating propertieseven for thin films.

(4) As demonstrated by the results reported in Table 3 for thecomparison in Example 7 of the insulating properties of an acrylic resinemulsion with those of a vinylidene chloride resin using the coatingmethod according to the present invention, at a withstand voltage of 500V the acrylic type gave the better volume resistivity of 1.6 to 4.8×10¹⁶ohm-cm, against the 3.0 to 4.0×10¹⁵ ohm-cm given by the vinylidenechloride type.

BENEFITS OF THE INVENTION

A method according to the present invention for coating the surface oflaminated motor cores provides an insulating layer with excellentinsulating properties and at the same time provides a shorter treatmentoperation than the prior insulating treatment operations. Moreover, theinvention method can provide a sufficiently thick coating in the cornerswithout having to raise the thickness of the insulating layer on otherparts of the coated substrate. As a result, the thickness of theinsulating layer on the surface of the coated substrate as a whole canbe reduced while at the same time an insulating film is obtained thathas excellent insulating properties.

These effects satisfy the requirements imposed by reductions in motorsize and thickness.

What is claimed is:
 1. A method for forming an electrically insulatingfilm on a motor core surface comprising the steps of: depositing anuncured resin film on electrically conductive projecting poles andinterior corners of a motor core by contacting said motor core with anautodepositing waterborne coating composition under autodepositionconditions sufficient to form an uncured resin film; and drying saiduncured resin film by heating to form an electrically insulating resinfilm on said poles.
 2. A method according to claim 1, wherein saidcoating-forming resin is a polymer or copolymer of at least one monomerselected from the group consisting of acrylic and methacrylic acids,esters of acrylic and methacrylic acids with alcohols having from 1 to 4carbon atoms per alcohol molecule, and nitrites and amides of acrylicand methacrylic acids.
 3. A motor core having an insulating film formedby the method of claim 2 on a plurality of projecting poles of saidmotor core.
 4. A method according to claim 1 wherein said autodepositingwaterborne coating composition comprises an emulsion of coating-formingresin, acid, oxidizing agent, metal ions and water.